The Sgs-4 locus in Drosophila melanogaster codes for a developmentally regulated protein made in large amounts in larval salivary glands and used as part of the pupal glue. This proposal suggests several ways to examine Sgs-4 expression. Genetic recombination between Sgs-4 mutants will be used to locate a regulatory region on the 5' side of Sgs-4. The genetic crossover points will be defined by Southern blot analysis of the recombinant chromosomes. The region defined this way will be examined in more detail by DNA sequence analysis of SGS-4 mutants. There is a tissue specific change in chromatin structure on the 5' side of Sgs-4 when the gene is active. This structural change is detected as a site in the chromatin which is hypersensitive to DNase I digestion. The involvement of this site in Sgs-4 expression will be examined by looking for it in the chromatin of Sgs-4 mutants and by determining when during development it appears. Formation of the hypersensitive site might affect Sgs-4 expression by altering the organization of nucleosomes near the 5' end of the gene. This possibility will be examined using partial micrococcal nuclease digestion to locate the nucleosomes 5' to Sgs-4 when the gene is active or inactive. Finally the protein and DNA sequence requirements for in vitro formation of a DNase hypersensitive site will be investigated.